Present day imprinting mechanisms, for example, electrophotographic copiers, laser printers, inkjet printers and the like, are generally capable of handling more than a single size of print media. An automatic manner of sensing the size of print media which has been loaded in a supply bin for inputting to a printing mechanism is the subject to be addressed herein. The print media may be any type of substantially flat material, such as plain paper, specialty paper, card-stock, fabric, transparencies, foils, mylar and the like, but the most common type or print media is paper. For convenience, I will discuss printing on paper as a representative example of these various types of print media. The media may be supplied to the printing mechanism in a variety of different sizes. For instance, in desktop inkjet or laser printers, paper is typically supplied in a stack of cut-sheets, such as letter size, legal size, or A-4 size paper, which are loaded or placed in a supply bin or input tray for subsequent input to the printing mechanism. Smaller sized envelopes or postcards or other media sizes may also be used for printing. Typically, the media sheets, cards or envelopes are sequentially pulled from the top of the stack and printed on, after which the printed sheets are deposited in an output tray.
It is desirable to have a printer which can measure and communicate what size media has been loaded in the input tray to the printer controller and to a host computer, particularly when the printer is not within sight of the computer user, such as when several computer users on a networked system share a printer or printers. Even when a printer is located on the users desktop, it may be helpful to provide the user with a warning if the wrong size media is loaded for a particular print job. This would tell the user to load the proper size media, or to adjust the print job parameters to fit the size of the loaded print media.
Automatic media size sensing must be highly accurate to be useful. In order to avoid toner contamination in the mechanism, this is particularly true for printing mechanisms which are capable of printing to the edges or near the edges of the sheets of print media. Sensing of media length and width is a feature often found in high-end printers and plotters for business and industrial use. However, these size sensing devices are typically quite cost-sensitive and general application sensing devices of the quality required can be very costly. Typically, for the small business and home markets where there is much more price sensitivity, automated media type detection has not been economically viable.
In the past, various methods have been used for media size determination. For example the use of a retroflective photo diode or a capacitance sensor mounted at some point in the printer paper path or on the print carriage to detect leading and trailing or side edges of the media sheet. However, retroflective and capacitive sensors are themselves expensive with the added direct and indirect costs incurred due to where within the printer the sensors are mounted and operate. Linear variable resistive and capacitive sensors mounted within the input paper tray tend to be less expensive than other methods, but, nevertheless, sensors of the required accuracy and sensitivity are expensive.
For example, one method of print media size detection utilizes a linear potentiometer mounted in the input paper tray. While this method provides a continuous output (i.e., a smooth function of output versus position), it can be either expensive, using large sensor components, or have lower accuracy due to scaling down the traverse with gears, levers or the like, and using smaller potentiometers. Another method is to use absolute encoders (optical, magnetic, etc, for example) to measure the true position of a movable member. In a printing mechanism supporting sizes of A-5 ledger media (222 mm of travel), an encoder would require 8 bits of resolution for 1 mm accuracy. Again, this can be expensive.
Accordingly there is a need for a sensor for determining what size print media is loaded in a printer input or supply tray having sufficient accuracy to be useful yet inexpensive enough to be utilized for low-end applications such as small business and home use.